1. Field of the Invention
The invention relates to methods for writing servo patterns in tapes.
2. Discussion
Some recording tapes include a thin layer, e.g., <150 nm, containing small, e.g., <60 nm, acicular magnetic metal particles. The particles are coated onto a plastic base film typically using a slightly thicker under layer to smooth the intrinsic roughness of the base film material. The particles in this type of media typically have a packing fraction of about 40%-60%. Tape manufacturers attempt to align the particles along a longitudinal tape direction when coating the tape. This results in a tape which has an in-plane magnetic moment and noise properties that are largely dictated by particle size, particle distribution, coating uniformity, and surface roughness.
The read back pulse width, pulse shape, and intrinsic signal to noise ratio of prerecorded servo patterns written in these tapes are limited by the tape's intrinsic recording characteristics. An example of such a servo read back signal is shown in FIG. 1. An enlarged portion of a pair of pulses of this signal is shown in FIG. 2. Timing based servo control systems use the timing and position of the peaks of the read back signal to determine the position of the head relative to the prerecorded servo pattern.
Other recording tapes include different types of particles, e.g., barium-ferrite or iron nitride, and coatings, e.g., advanced metal evaporated or sputtered. The use of these particles and coatings may increase linear density because the magnetic transitions between bits of data written as regions of opposite magnetic polarity are much narrower than those in metal particle tape. Due to the nature of these types of coatings, however, conventional methods used to write pre-recorded servo patterns produce large baseline offsets between the written transitions as well as marked asymmetry in the pulse shapes as shown in FIG. 3. An enlarged portion of a pair of pulses of this signal is shown in FIG. 4.